The present invention relates generally to the control of hydraulic systems, and particularly to a model based pressure control strategy for a hydraulic system with a variable delivery pump.
Hydraulic systems, particularly those used in conjunction with an internal combustion engine, have been known for years. For example, Caterpillar Inc. of Peoria, Illinois has been successfully manufacturing and selling hydraulic fuel injection systems for many years. In the past, these systems typically included at least one common rail containing high pressure actuation fluid that was supplied to actuate a plurality of hydraulic devices such as hydraulically actuated fuel injectors and/or gas exchange valve actuators (engine brake, intake, exhaust). The high pressure common rail was supplied with pressurized actuation fluid by a fixed displacement pump. Control of pressure in the common rail was maintained by sizing the pump to always supply more than the needed amount of high pressure fluid and then utilizing a rail pressure control valve to spill a portion of the fluid in the common rail back to the low pressure reservoir. The control system strategy for these systems typically relied upon a feedback control loop in which the desired rail pressure was compared to the measured or estimated rail pressure, and the position of the rail pressure control valve was set as a function of the error signal generated by that comparison. A system of this type is illustrated, for example, in U.S. Pat. No. 5,357,912 to Barnes et al. While these hydraulic systems, and the control thereof, have performed magnificently for many years, there remains room for improvement.
One area in which these previous hydraulic systems could be improved is by decreasing the amount of pressurized actuation fluid that is spilled back to the low pressure reservoir without performing any useful work, such as actuating one of the hydraulic devices. In other words, energy is consumed and arguably wasted whenever the rail pressure control valve opened to allow pressurized fluid from the high pressure rail to leak back to the low pressure reservoir. In order to decrease the amount of energy consumed in controlling the pressure in the hydraulic system, one strategy has been to introduce a variable delivery pump and eliminate the previous rail pressure control valve. Such a hydraulic system is shown and described in co-owned U.S. Pat. No. 6,035,828 to Anderson et al. This system greatly reduces the amount of wasted energy since the pump is controlled to produce only the amount of actuation fluid necessary to maintain a desired rail pressure. Although this type of fluid supply and pressurization strategy has considerable promise, it still may suffer from at least one subtle drawback when it is controlled via a feedback loop based upon a comparison of the desired rail pressure to the actual rail pressure. Due at least in part to the fact that the fluid being consumed from the high pressure common rail can be rapidly and continuously changing, engineers have observed that the control system can be at least temporarily overwhelmed in this highly dynamic system. In other words, the system can sometimes demonstrate an inability to both maintain an adequate fluid supply to the hydraulic devices and do so at the desired pressure without unacceptable lags between the control system response and the fluid demands of the hydraulic devices.
The present invention is directed to these and other problems associated with hydraulic systems.
In one aspect, a method of controlling a hydraulic system includes at least some features of the previous control systems based upon a pressure error feedback control system. Thus, the method includes a step of generating a control variable at least in part by comparing a desired liquid pressure to an estimated liquid pressure. Next, the liquid consumption rate of the hydraulic system is estimated. Finally, the pump output rate is set as a function of the control variable and the estimated system consumption rate.
In another aspect, a method of controlling liquid pressure in a common rail hydraulic system for an engine includes a step of estimating engine speed, the viscosity of the liquid in the hydraulic system and the rail pressure of the hydraulic system. The injector consumption rate and the pump consumption rate are also estimated. A control rate is generated at least in part by comparing the desired rail pressure to the estimated rail pressure. Finally, the pump output rate is set as a function of the control rate plus the estimated injector consumption rate plus the estimated pump consumption rate.
In still another aspect, a common rail hydraulic system includes a variable delivery pump with an outlet. At least one hydraulic device has an inlet. A common rail has an inlet fluidly connected to the outlet of the variable delivery pump, and an outlet connected to the inlet of the at least one hydraulic device. A pump output controller is operably coupled to the variable delivery pump, and produces a pump control signal that is a function of a desired rail pressure, an estimated rail pressure and an estimated consumption rate of the hydraulic system.